The present invention relates generally to filters for electrical signals, more particularly to a narrow band filter using frequency-dependent L-C components, and still more particularly to a super-narrow-band filter on the order of 0.05% which utilizes frequency-dependent L-C components and which is constructed of superconducting materials.
Narrow-band filters are particularly useful in the communications industry and particularly for cellular communications systems which utilize microwave signals. At times, cellular communications have two or more service providers operating on separate bands within the same geographical area. In such instances, it is essential that the signals from one provider do not interfere with the signals of the other provider(s). At the same time, the signal throughput within the allocated frequency range should have a very small loss.
Additionally, within a single provider""s allocated frequency, it is desirable for the communication system to be able to handle multiple signals. Several such systems are available, including frequency division multiple access (FDMA), time division multiple access (TDMA), code division multiple access (CDMA), and broad-band CDMA (b-CDMA). Providers using the first two methods of multiple access need filters to divide their allocated frequencies in the multiple bands. Alternatively, CDMA operators might also gain an advantage from dividing the frequency range into bands. In such cases, the narrower the bandwidth of the filter, the closer together one may place the channels. Thus, efforts have been previously made to construct very narrow bandpass filters, preferably with a fractional-band width of less than 0.05%.
An additional consideration for electrical signal filters is overall size. For example, with the development of cellular communication technology the cell size (e.g., the area within which a single base station operates) will get much smallerxe2x80x94perhaps covering only a block or even a building. As a result, base station providers will need to buy or lease space for the stations. Each station requires many separate filters. The size of the filter becomes increasingly important in such an environment. It is, therefore, desirable to minimize filter size while realizing a filter with very narrow fractional-bandwidth and high quality factor Q. In the past, however, several factors have limited attempts to reduce the filter size.
For example, in narrow-band filter designs, achieving weak coupling is a challenge. Filter designs in a microstrip configuration are easily fabricated. However, very-narrow-bandwidth microstrip filters have not been realized because coupling between the resonators decays only slowly as a function of element separation. Attempts to reduce fractional-bandwidth in a microstrip configuration using selective coupling techniques have met with only limited success. The narrowest fractional-bandwidth reported to date in a microstrip configuration was 0.6%. Realization of weak coupling by element separation is ultimately limited by the feedthrough level of the microstrip circuit.
Two other approaches have been considered for very-narrow-bandwidth filters. First, cavity type filters may be used. However, such filters are usually quite large. Second, filters in stripline configurations may be used, but such devices are usually hard to package. Therefore, by utilizing either of these two types of devices there is an inevitable increase in the final system size, complexity and the engineering cost.
Accordingly, there exists a need for a super-narrow-bandwidth filter having the convenient fabrication advantage of microstrip filters while achieving, in a small filter, the equivalent of the very weak coupling necessary for a super-narrow fractional bandwidth. This objective may be achieved by utilizing a frequency-dependent inductor-based design to achieve the equivalent of very weak coupling.
The present invention provides for a super-narrow band filter using frequency dependent L-C components. The invention utilizes a frequency dependent L-C circuit with a positive slope k for the inductor values as a function of frequency. The positive k value allows the realization of a very narrow-band filters. Although the example of communications and cellular technology is used herein, such application is only one of many in which the principles of the present invention may be employed. Accordingly, the present invention should not be construed as limited by such examples.
In a preferred embodiment filter, the filter is designed to meet a predetermined transmission response of S21 which can be expressed in terms of ABCD matrix parameters:       S    21    =            2      ⁢                                    Z            1                    ⁢                      Z            2                                                        Z          2                ⁢        a            +      b      +                        Z          1                ⁢                  Z          2                ⁢        c            +                        Z          1                ⁢        d            
where Z1 and Z2 are input and output impedances; a and d are pure real numbers; and b and c are pure imaginary numbers. As set forth in more detail below, the real numbers a and d depend on the variable Lxcfx892 (e.g., the inductance times the frequency squared, a well known variable in the art). A frequency transformation may then be introduced which keeps Lxcfx892 invariant (discussed in further detail below). Thus, a and d, which contribute to the real part of the denominator in S21, will remain unchanged. As set forth in more detail below, the imaginary numbers b and c depend on the variable jxcfx89 (e.g., the imaginary number times the frequency, a well known variable in the art). Furthermore, if changes caused by the frequency transformation due to the jxcfx89 part in b and c are small enough (which is exactly equal to zero at the filter passband center, xcfx890), then the imaginary part of the denominator in S21 will remain invariant also. Accordingly, the whole transmission response S21 will remain unchanged after the frequency transformation.
With the availability of high temperature superconductors, filters with circuit Qs of 40,000 are now possible. The present invention, when realized in a high Q embodiment enables super-narrow-band filters not previously possible.
The various features of the present invention include several advantages over prior lumped-element approaches. By way of example, the methodology of the present invention offers very large equivalent values of planar lumped-element inductors without requiring the cross-over of thin films. It also shrinks the filter bandwidth without further reduction of the weak coupling. Third, it saves more wafer area than conventional lumped-element circuits for the same circuit performance.
It will also be appreciated by those skilled in the art that this invention has wide application in narrow-band circuits. For example, the invention may be used to realize very narrow-band filters; realize large effective values of inductance for narrow-band applications such as DC-bias inductors that block high frequency signals; realize lumped-element circuits with even smaller areas; introduce additional poles for bandpass and low-pass filters; and be used in applications in other high-Q circuits such as superconductor applications.
Therefore, according to one aspect of the present invention, there is provided a narrow-band filter apparatus using frequency transformation, comprising: (a) a capacitive element and (b) an inductive element having an effective inductance and operatively connected to said capacitive element, wherein said effective inductance increases as a function of frequency.
According to another aspect of the invention, there is provided a bandpass filter, comprising: a plurality of L-C filter elements, each of said L-C filter elements comprising an inductor, the inductor having an initial and an effective inductance, and a capacitor in parallel with the inductor, wherein the effective inductance of each of the L-C filter elements is larger than the initial inductance of said inductor and increases with increases in frequency; and a plurality of uncapacitive elements interposed between the L-C filter elements, whereby a lumped-element filter is formed.
These and other advantages and features which characterize the present invention are pointed out with particularity in the claims annexed hereto and forming a further part hereof. However, for a better understanding of the invention, the advantages and objects attained by its use, reference should be made to the drawing which forms a further part hereof, and to the accompanying descriptive matter, in which there is illustrated and described preferred embodiments of the present invention.